Threading taps are well known in the prior art and comprise longitudinally extending flutes forming lands therebetween, the lands being provided with screw-cutting teeth. In use of such taps considerable care had to be exerted in the threading operation to avoid breaking the tap. It was customary to use cutting oil during the threading cutting operation and, even so, it was necessary to back off the tap one or more times to remove chips which otherwise might cause the tap to bind and break.
Attempts have been made in the prior art to overcome breaking of taps and these included the cutting of cross-grooves in the lands. However, these attempts were not considered practical since in some only a tooth was removed to form the groove or in others the groove did not extend deep enough and therefore did not provide adequate chip clearance. Still in others, the grooves were deep enough but were in radial alignment and therefore not only weakened the tap but caused stringing of the chips and consequent breakage of the tap.
In my improved tap, the cross-grooves in the lands are of a length equal to at least two teeth; have a depth greater than the root depth of the adjoining teeth; and are in staggered relation so that no transverse portion of the tap is material weakened. Also, because the cross-grooves are staggered, no stringing occurs since the chips are broken up between lands. In addition, I have positioned certain of the cross-grooves to define maximum and minimum limits for threading the tap to produce screw threads of a required number and depth in accordance with good machine shop practice.